378 AIM AND TKOGRESS OF PHYSICAL SCIENCE. 



Thus, then, we obtain mechanical motive force from 

 the most varied processes of nature in the most different 

 ways; but it will also be remarked in only a limited 

 quantity. In doing so we always consume something that 

 nature supplies to us. In the watermill we use a quantity 

 of water collected at an elevation, coal in the steam- 

 engine, zinc and sulphuric acid in the electro-magnetic 

 machine, food for the horse ; in the windmill we use up 

 the motion of the wind, which is arrested by the sails. 



Conversely, if we have a motive force at our disposal 

 we can develop with it forms of action of the most varied 

 kind. It will not be necessary in this place to enumerate 

 the countless diversity of industrial machines, and the 

 varieties of work which they perform. 



Let us rather consider the physical differences of the 

 possible performance of a motive power. With its help 

 we can raise loads, pump water to an elevation, compress 

 gases, set a railway train in motion, and through friction 

 generate heat. By its aid we can turn magneto-electric 

 machines, and produce electric currents, and with them 

 decompose water and other chemical compounds having 

 the most powerful affinities, render wires incandescent, 

 magnetise iron> &c. 



Moreover, had we at our disposal a sufficient me- 

 chanical motive force we could lestore all those states 

 and conditions from which, as was seen above, we are 

 enabled at the outset to derive mechanical motive power. 



As, however, the motive power derived from any 

 given natural process is limited, so likewise is there a 

 limitation to the total amount of modifications which we 

 may produce by the use of any given motive power. 



These deductions, arrived at first in isolated instances 

 from machines and physical apparatus, have now been 

 welded into a law of nature of the widest validity. Every 

 thange in nature is equivalent to a certain development, 



